1. Field of the Invention
This invention relates to a zoom lens, and, more particularly to a zoom lens exhibiting high variable power, excellent optical performance, and a reduced size.
2. Description of the Related Art
Hitherto, a variety of zoom lenses of so-called a rear focusing type for a photographing camera or a video camera have been disclosed, the rear focusing type structure being characterized in that lens groups except for a first lens group adjacent to the object are moved for the purpose of performing the focusing.
In general, since the rear focusing type zoom lens conducts its focusing by moving lens groups of a relatively reduced size and lightened weight, it exhibits advantages in that the force required to move the lens groups can be reduced and a quick focusing can be conducted.
For example, a so-called a four-group zoom lens of the rear focusing type comprises four lens groups and is structured in such a manner that the third lens group of the four lens groups is moved for the purpose of conducting the focusing, the four lens group consisting of, counted from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power which is capable of varying the magnification, a third lens group having a negative refractive power which is capable of correcting the image change due to the magnification variance, and a fourth lens group having a positive refractive power.
Another zoom lens of the rear focusing type has been disclosed in Japanese Laid-Open Patent Application No. Sho 58-136012, this zoom lens being structured in such a manner that the magnification varying portion thereof comprises three or more lens groups, and a portion of these groups is arranged to be moved for the purpose of conducting the focusing.
However, the above-described type zoom lenses encounter the following problem:
the distance of the extension distance of the focusing lens group with respect to the same object distance becomes variable due to the difference in the zoom position, that is, the difference in the focal distance, and this extension distance is varied in the form of a quadratic curve or is varied discontinuously;
if the magnification varying ratio is raised, the wide-angle end needs to be given a larger space in which the focusing lens group moves, causing the size of the lens system to be enlarged;
in the rear focusing type zoom lens, the extension distance of the focusing lens group with respect to the same object distance becomes considerably longer at the telephoto end than that at the wide-angle end;
the distance of movement of the image plane with respect to the distance of movement of the focusing lens group, that is, the sensitivity becomes larger at the telephoto end, and a certain amount of increase in the sensitivity causes a difficulty in performing the mechanical control of movement of the focusing lens group;
if the sensitivity at the telephoto end is, on the contrary, arranged to a degree which is capable of being controlled, the sensitivity at the wide-angle end becomes reduced excessively, causing a necessity of providing a large space in which the focusing lens group moves, and the size of the lens group becomes large.
There have been related arts disclosed in U.S. patent application Ser. No. 026,004 filed on Mar. 16, 1987. Furthermore, related arts disclosed in U.S. Pat. No. 3,501,224, and that disclosed in U.S. Pat. No. 4,256,381 are known.